In a field study, 39 current and former workers engaged in the manufacture of decabromobiphenyl (PBB) and decabromobiphenyl oxide (PBBO) and 96 control workers (electricians/linemen), steel workers, and community workers) were examined in August, 1978. Included were independent interviews of occupational and medical history, general physical, dermatological and neurological examinations, nerve conduction tests and blood and urine tests. Four of the PBB/PBBO workers and none of the control workers were found to be hypothyroid (P less than 0.006). It is proposed to conduct another field study of an enlarged cohort of workers from this plant focussing on thyroid function tests and on reproductive function tests (follicle stimulating hormone (FSH)). The cohort will include all workers engaged in the manufacture of PBB/PBBO at this plant from January 1, 1973 to July 1, 1980. It is expected that 170 (68 percent) of the estimated total 250 PBB/PBBO workers will be recruited, including reexaminees. In addition, 100 persons will be selected from control workers groups, including reexaminees. Expert industrial hygiene methods will be used to provide estimates of grades of exposure to the substances of interest. The specific aims are to determine: a) if PBB, PBBO or PBB/PBBO together are associated with thyroid health effects and/or reproductive endocrine effects, and b) if so, (1) whether there is a relation between approximate grades of exposure and health effects, and (2) whether there is a time-dependent (induction/chronicity) health effect.